Transition temperatures and upper critical fields of NbN thin films fabricated at room temperature

Abstract

NbN thin films were deposited on thermally oxidized Si substrate at room temperature by using reactive magnetron sputtering in an Ar-N 2 gas mixture. Total sputtering gas pressure was fixed while varying N 2 flow rate from 1.4 sccm to 2.9 sccm. X-ray diffraction pattern analysis revealed dominant NbN(200) orientation in the low N 2 flow rate but emerging of (111) orientation with diminishing (200) orientation at higher flow rate. The dependences of the superconducting properties on the N 2 gas flow rate were investigated. All the NbN thin films showed a small negative temperature coefficient of resistance with resistivity ratio between 300 K and 20 K in the range from 0.98 to 0.89 as the N 2 flow rate is increased. Transition temperature showed non-monotonic dependence on N 2 flow rate reaching as high as 11.12 K determined by the mid-point temperature of the transition with transition width of 0.3 K. On the other hand, the upper critical field showed roughly linear increase with N 2 flow rate up to 2.7 sccm. The highest upper critical field extrapolated to 0 K was 17.4 T with corresponding coherence length of 4.3 nm. Our results are discussed with the granular nature of NbN thin films.

Suppression of superconductivity in superconductor/ferromagnet multilayers

“…Except those peaks, no other discernable peak was observed in all samples. NbN (200) and (111) peaks used to be detected at 41 and 35, and (110) peak of -Fe used to exist at around 44 when polycrystalline phases were formed [13,14]. The present results of the XRD scans state that all NbN and FeN layers prepared with the current deposition conditions are amorphous in phase.…”

“…In general, the effects of boundary scattering and reduced dimensionality compared to the superconducting coherence length cause T c to fall with decreasing d s . For a reference, the Ginzburg-Landau coherence length at 0 K estimated from the upper critical field was ~4.5 nm for similarly prepared NbN films [13]. The largest T c suppression has been observed in the F/S/F structure for a given d s .…”

“…The details of the fabrication conditions for each layer have been published elsewhere [13,14]. The base pressure of the multi-target sputtering system after overnight pumping was lower than 310 -7 Torr.…”